The young of many species vocalize when separated from the mother. The ability of opiate antagonists to block the quieting provided by either opioid agonists or a social companion implicates the opioid system in the isolation response. Vocalization and other behaviors will be monitored after administration of specific opiate receptor agonists to isolated pups, or of opiate antagonists to pups tested in the comforting presence of a companion. By testing at three ages and by varying the duration of isolation, we will study the development and time course of opioid modulation of distress behavior. Experiments will be repeated as each new technique is mastered, so that the same behavioral manipulations will be used in association with quantitative autoradiography for the study of specific opioid receptor subtypes, radioimmunoassay and immunocytochemistry to examine endogenous opioid peptides, and Northern Blot analyses and in situ hybridization to investigate messenger RNA activity. MacLean (1985) characterizes the separation call as being perhaps the earliest and most basic mammalian vocalization and Bowlby (1969, 1973) speculates that separation anxiety is not learned by a child but is an evolved protest mechanism, released by the mother's absence. The distress cry of the isolated pup, therefore, may provide a model for the intense anxiety common to mammalian young when faced with the sudden loss of familiar social figures. If we can understand the neurochemical and neuroanatomical substrates of this 'normal' manifestation of early anxiety, it may provide us with clues to the etiology of childhood separation disorders in humans. Understanding how the opioid system is intended to function may help improve treatment of infants made dysfunctional by maternal drug abuse.